Is NAS6603-3 the same as NAS1303-3?
Yes. The NAS1303 has been superceded to NAS6603. They meet the same dims and specifications per the tech data sheet.
What is the NAS6603-XX equivalent for NAS1303-5H?
NAS1303-5H has a drilled head, per the "H" in the part number. We do not stock a drilled head unit, but the unit without the drilled head is NAS6603-5, which we do stock and can ship right away.